Purpose: A 4-Pi non-coplanar converging (4PiNC) modality has been actively developed in our institution, in which the radiation beams are delivered along conic trajectories and rotate around the midline of a patient. The proposed design eliminates the potential gantry-couch collision, a major concern of non-coplanar beam delivery. This study introduces a method to implement the 4PiNC planning in the public-domain treatment planning system (TPS) PLUNC.

Methods:The cone angle of a 4PiNC delivery is defined as the angle between the cone edge and the transverse iso-plane. The beam angle defines its relative position on the cone. We have mathematically proved that there is a one-to-one correspondence between a 4PiNC beam specification (cone and beam angle) and a LINAC beam specification (gantry and couch angle). Therefore, a graphic interface was written in PLUNC to design a 4PiNC RT plan using existing commissioned LINAC machines. The method was validated visually on a spherical phantom of 15cm radius with 12 beams evenly distributed on a cone, and with cone angle 40, -30, 20, -10 and 0 degree respectively. A 4PiNC RT plan was made for a prostate SBRT case with the afore-mentioned 5 cones (20 beams on each cone) and was compared with the conventional five-beam IMRT plan.

Results:In the phantom study, the transverse planes at -3, 0 and 3 cm from the iso-plane as well as the sagittal iso-plane were examined; the beam separations and cone angles were consistent with those specified. The prostate study suggested that the OAR mean dose can be reduced by up to 44.5% by the 4PiNC RT plan with comparable PTV coverage.

Conclusion:The RT planning for the proposed 4PiNC delivery system can be implemented in a TPS for conventional LINAC beams. This allows dosimetric investigation of the proposed system even before finalizing the hardware design.

Funding Support, Disclosures, and Conflict of Interest: This work was partially supported by a DoD grant DoD W81XWH1010862